This invention relates to coated glass fiber bundles suitable for rubber reinforcement, and more particularly relates to glass fiber cord having substantially complete individual filament encapsulation with a resorcinol formaldehyde and elastomer coating composition containing a material which retards the degradation of the coated cord during exposure to heat and moisture.
It has long been recognized that glass fiber material makes an ideal reinforcement for rubber products such as automobile tires and the like. In preparing glass fiber material for such applications, the individual glass fibers and groups of glass fibers in the form of strand, rope, cord, roving, fabric and the like are coated with a rubber adhesive to aid in bonding the glass to the elastomeric material to be reinforced. The rubber adhesive generally comprises a resin and an elastomeric material to link between the glass and the main body of material being reinforced. Generally, in the production of fiber glass reinforcing cords or other bundle forms, individual fibers are coated with a sizing and then the fibers are brought together in bundle form. The bundle is then coated by dipping or otherwise contacting it with a coating mixture containing an elastomeric latex and a homogeneous resinous component. Commonly, the sizing contains a coupling agent such as a silane, a lubricant and other ingredients to assist in the handling of the cord during processing.
The term "elastomer", as used herein, is intended to mean and include both synthetic and natural rubber. "Natural rubber", as used herein, is the elastic solid obtained from the sap or latex of the Hevea tree, the major constituent being the homopolymer of 2-methyl-1,3-butadiene (isoprene). "Synthetic rubber", as used herein, is meant to encompass polymers based upon at least 2 percent of a conjugated unsaturated monomer, the conjugation being in the 1 to 3 position in the monomer chain and the final polymer in its uncured state having an extensibility of at least 200 percent and a memory of at least 90 percent when stretched within the extensibility limits and released instantaneously. The conjugated, unsaturated monomers which are used in the preparation of synthetic rubber are, but are not limited to, chloroprene, butadiene, isoprene, cyclopentadiene, dicyclopentadiene and the like. Other olefins capable of free radical, anionic, or cationic interpolymerization into the polymer chain with the conjugated unsaturated monomer are useful in forming synthetic rubbers. These olefins are typically monoethylenically unsaturated monomers. Monoethylenically unsaturated as used herein is characterized by the monomer having one CH=C&lt; group. These monoethylenically unsaturated monomers are, but not limited to, the acrylic monomers such as methacrylic acid, acrylic acid, acrylonitrile, methacrylonitrile, methylacrylate, methylmethacrylate, ethylacrylate, ethylmethylacrylate and the like; monoolefinic hydocarbons such as ethylene, butylene, propylene, styrene, alpha-methylstyrene and the like; and other functional monounsaturated monomers such as vinylpyridine, vinylpyrollidone and the like functional vinylic monomers.
Glass fibers are excellent reinforcing materials and are distinguishable from other fibrous reinforcing materials such as natural and synthetic organic fibers in that glass fibers do not become elongated or deformed under stress to the extent that other fibers do. Unlike other fibers, particular combinations of glass fibers with encapsulating coatings cooperate to yield reinforcing materials that have greater tensile strength than either the glass or coating material alone. While other materials, which are subject to substantial stress elongation, are essentially limited in tensile strength to the basic strength of the bare fibers, even if coated, properly coated glass fibers have greater strength than the glass alone. For example, the low modulus of elasticity of glass may be exploited to provide reinforced tires having superior road performance if an appropriate coating medium is provided to transfer stresses to all fibers in the glass fiber cord so that loading throughout is substantially uniform. This phenomenon is illustrated by the observation that a typical, uncoated glass fiber cord (G-75, 5/0, filament count 2,000 i.g. 2,000 filament strands of G fibers of about 38 .times. 10.sup..sup.-5 inch diameter, 7,500 yards per pound) has a tensile strength of about 35 to 40 pounds by ASTM test D578-52, but, when coated with a resorcinol formaldehyde latex coating, such a cord has a tensile strength of about 50 to 70 pounds.
Unfortunately, when exposed to a warm, moist environment, coated glass fiber cords lose strength. After about 1 week of exposure at 120.degree.F. and 95 percent relative humidity, the coated cord described above has a strength of only 35 to 40 pounds. The strength degradation is observed even when the exposure is in a typical warehouse which is dark and not affected by any particular oxidizing atmosphere. The problem is surprising in view of the fact that rubber and elastomer materials themselves are not found to degrade significantly in warm, moist environments absent the influence of ultraviolet light, ozone and higher temperatures. The loss of strength in resorcinol formaldehyde elastomer coated glass fiber cord does not appear to be explained as conventional oxidation for it would be expected that a substantial amount of resorcinol formaldehyde (which has antioxidative characteristics) would protect the elastomer from degradation. It has been found that the addition of typical antioxidants such as phenols, amines and the like do not protect the elastomer adequately from such degradation.
During the course of experimenation, it was found that a material containing 2,6-ditert-butyl-4-phenylphenol absorbed on a diatomaceous earth imparted moisture resistance to the adhesive coated glass fiber bundles in moist environments when used at a 0.4 percent by weight level based on the total elastomer content of the coating composition. This product containing the above phenolic compound and the diatomaceous earth has found much commercial success. Naturally, it was assumed that the phenolic compound was responsible for this improvement in tensile strength after aging due to its known antioxidative and antidegradative properties. However, further investigation surprisingly revealed that the improvement in the aged bundles' tensile strength was due to the treated diatomaceous earth.
It has been necessary in the past to avoid the degradation of elastomer coated glass fiber materials by providing a moisture barrier such as a polyethylene bag about them and providing a desiccant such as a silica gel with the stored materials. Even when this expensive protective storage is employed, it is necessary to expose the coated glass fiber cord during processing such as weaving to moisture laden atmosphere and degradation can then occur. This is an expensive and unsatisfactory solution to the problem. As is described below, a satisfactory solution has been discovered.